The present invention relates to a displacement control valves for a variable displacement compressors used in a vehicle air conditioners.
A typical variable displacement compressor includes a crank chamber to accommodate a cam plate. The crank chamber is connected to a suction pressure zone by a control passage. The crank chamber pressure is adjusted for changing the inclination of the cam plate, which varies the compressor displacement. The crank chamber is connected to a discharge pressure zone by a supply passage. The supply passage supplies highly pressurized refrigerant gas to the crank chamber. Also, blowby gas is supplied to the crank chamber. A displacement control valve is located in the control passage. The position of the control valve, or its opening size, is changed to regulate the amount of refrigerant gas supplied from the crank chamber to the suction pressure zone, which alters the crank chamber pressure.
Japanese Unexamined Patent Publication No. 6-26454 discloses such a displacement control valve for compressors. The valve of the publication is illustrated in FIGS. 8 and 9. The valve includes a valve chamber 101, which is connected to a crank chamber of a compressor by a valve hole 102 and an upstream portion of a control passage. The valve chamber 101 is also connected to a suction pressure zone by a downstream portion of the control passage. A valve body 103 is housed in the valve chamber 101 to regulate the opening size of the valve hole 102. A bellows 104 is accommodated in the valve chamber 101. The bellows 104 is coupled to the valve body 103.
When the pressure in the valve chamber 101 is higher than a target value (target pressure), the bellows 104 contracts and moves the valve body 103 in a direction to open the valve hole 102. Accordingly, the amount of refrigerant gas flowing from the crank chamber to the suction pressure zone is increased, which lowers the crank chamber pressure. As a result, the compressor displacement is increased. When the pressure in the valve chamber 101 is lower than the target pressure, the bellows 104 expands and moves the valve 103 in a direction to close the valve hole 102. This decreases the amount of refrigerant gas flowing from the crank chamber to the suction pressure zone, which increases the crank chamber pressure. As a result, the compressor displacement is decreased. As described below, the target pressure is changed by altering the level of current supplied to a coil 108.
A plunger chamber 105 is defined in the control valve. A fixed core 106 is located in the plunger chamber 105. A plunger 107 is accommodated in the plunger chamber 105 and is located between the fixed core 106 and the valve chamber 101. The plunger 107 is coupled to the valve body 103. The coil 108 is located about the plunger chamber 105 and is located radially outward of both the fixed core 106 and the plunger 107.
When a current is sent to the coil 108, the plunger 107 is attracted to the fixed core 106. The attraction opposes, or reduces, the force that moves the valve body 103 in the direction to open the valve hole 102. The attraction thus raises the target pressure. The target pressure is increased when the current to the coil 108 is increased and the attractive force between the fixed core 106 and the plunger 107 is increased. The target pressure is maximized when the current to the coil 108 is maximized. The target pressure is decreased when the current to the coil 108 is decreased and the attractive force between the fixed core 106 and the plunger 107 is decreased. The target pressure is minimized when the current to the coil 108 is stopped.
The compression load of the compressor is great when the compressor is operating at a large displacement. If the engine speed is increased when the compressor is operating at a large displacement, the moving parts of the compressor will receive a great load. The compressor is connected to an external refrigerant circuit, which includes a condenser. If the condenser is not sufficiently cooled while the compressor is operating at a large displacement, the discharge pressure will be abnormally high. As a result, the compression load will be excessive, which increases the load on the moving parts.
In order to reduce the excessive load on the compressor, a clutch, which is located between the engine and the compressor, may be disengaged to stop the compressor. However, it is preferred that the vehicle air conditioner continue running to maintain a minimum cooling performance for the comfort of the passengers. Therefore, when the load on the compressor is excessive, the current to the coil 108 is maximized to maximize the target pressure. As a result, the compressor operates at the minimum displacement and the load on the compressor is reduced. Further, the air conditioner continues operating at a minimum performance level.
However, when the current to the coil 108 is stopped, the target pressure is minimized. In other words, when the target pressure is maximized, the current to the coil 108 must continue. Thus, if current cannot be sent to the coil 108 because of, for example, a broken wire, the target pressure is fixed to the minimum value. As a result, excessive loads on the compressor cannot be reduced. Also, even if the compressor is not operating under an excessive load, the displacement is unnecessarily increased if current cannot be sent to the coil 108, which abnormally increases the load on the compressor.